import Anthropic from '@anthropic-ai/sdk'

/**
 * Netlify Functions v2 handler.
 *
 * Proxies a request from the React frontend to a Claude Console-defined
 * Managed Agent via the /v1/sessions endpoints. The agent's model, system
 * prompt, tools, MCP servers, and skills are configured in the Console and
 * referenced here by ID.
 *
 * Wire protocol (downstream to the browser): newline-delimited JSON
 * (`application/x-ndjson`). One JSON object per line; the React side
 * parses incrementally as bytes arrive. Object shapes:
 *
 *   {"type":"text","text":"...markdown..."}        // append to brief
 *   {"type":"status","kind":"thinking"}
 *   {"type":"status","kind":"tool_use","name":"...","label":"..."}
 *   {"type":"status","kind":"tool_result","ok":true}
 *   {"type":"status","kind":"tool_result","ok":false,"message":"..."}
 *   {"type":"status","kind":"session_error","message":"..."}
 *   {"type":"heartbeat"}                            // keep-alive only
 *   {"type":"done"}                                 // session terminated cleanly
 *   {"type":"error","message":"..."}                // unrecoverable
 *
 * Reconnect strategy: the upstream Anthropic SSE stream
 * (`GET /v1/sessions/{id}/events/stream`) is not guaranteed to stay open
 * for the full life of a multi-turn session — long inter-turn gaps (e.g.
 * while MCP tools or the next model request are running) can trigger
 * upstream / proxy read timeouts. When the upstream iterator ends without
 * a terminal `session.status_*` event we treat it as a drop, fetch any
 * events we missed via `sessions.events.list()` (deduped by event id),
 * reopen the live stream, and keep going — up to a wall-clock budget.
 */

// All Managed Agents endpoints require this beta header.
const MANAGED_AGENTS_BETA = 'managed-agents-2026-04-01'

// Downstream keep-alive cadence. The browser's fetch reader will time
// out / appear stuck if no bytes flow for too long; we emit a
// `{"type":"heartbeat"}` line when the upstream is quiet.
const HEARTBEAT_MS = 10_000

// Hard ceiling on total wall-clock time the function will keep
// reconnecting upstream on behalf of a single browser request. Past this
// we emit an `error` line and close the stream so the user can retry
// rather than having the request hang indefinitely.
const RECONNECT_BUDGET_MS = 20 * 60 * 1000 // 20 minutes

// Short backoff between an upstream drop and the next reconnect attempt,
// so a tight error loop can't hammer the API.
const RECONNECT_BACKOFF_MS = 500

// ---------------------------------------------------------------------------
// Anthropic client (reused across warm Lambda invocations)
// ---------------------------------------------------------------------------
let _client
function getClient() {
  if (_client) return _client
  if (!process.env.ANTHROPIC_API_KEY) {
    throw new Error('Server is missing ANTHROPIC_API_KEY')
  }
  _client = new Anthropic({
    apiKey: process.env.ANTHROPIC_API_KEY,
    defaultHeaders: { 'anthropic-beta': MANAGED_AGENTS_BETA },
  })
  return _client
}

// A `session.status_idle` with `stop_reason: requires_action` means the
// agent is waiting on a client-side response (custom tool result, tool
// confirmation). For this app we don't expose custom tools, so it should
// never fire — but if it ever does we explicitly do NOT treat it as
// terminal and let the loop keep tailing.
function isTerminal(event) {
  if (event?.type === 'session.status_terminated') return true
  if (event?.type === 'session.status_idle') {
    const t = event.stop_reason?.type
    return t === 'end_turn' || t === 'retries_exhausted'
  }
  return false
}

function describeToolUse(event) {
  const name = event.name || 'tool'
  const input = event.input || {}
  if (name === 'web_search' && input.query) {
    return `Searching the web: ${input.query}`
  }
  if (name === 'web_fetch' && input.url) {
    return `Fetching: ${input.url}`
  }
  if (event.type === 'agent.mcp_tool_use') {
    const args = Object.entries(input)
      .filter(([k]) => k !== 'limit')
      .slice(0, 3)
      .map(([k, v]) =>
        typeof v === 'object' ? `${k}=…` : `${k}=${String(v).slice(0, 40)}`,
      )
      .join(', ')
    return `${name}${args ? ` (${args})` : ''}`
  }
  return name
}

// ---------------------------------------------------------------------------
// Handler
// ---------------------------------------------------------------------------
export default async (req /*, context */) => {
  if (req.method !== 'POST') {
    return new Response('Method Not Allowed', { status: 405 })
  }

  let payload
  try {
    payload = await req.json()
  } catch {
    return new Response('Invalid JSON body', { status: 400 })
  }

  const {
    stationName = '',
    stationLocation = '',
    stationWebsite = '',
  } = payload || {}

  // Server-side URL validation (mirrors the client-side regex).
  const URL_REGEX =
    /^(https?:\/\/)?([\w-]+\.)+[\w-]{2,}(\/[\w\-._~:/?#[\]@!$&'()*+,;=%]*)?$/i
  if (!URL_REGEX.test(stationWebsite)) {
    return new Response('Invalid station website URL', { status: 400 })
  }

  const AGENT_ID = process.env.ANTHROPIC_AGENT_ID
  const ENVIRONMENT_ID = process.env.ANTHROPIC_ENVIRONMENT_ID
  const VAULT_IDS = (process.env.ANTHROPIC_VAULT_IDS || '')
    .split(',')
    .map((s) => s.trim())
    .filter(Boolean)

  const missing = []
  if (!AGENT_ID) missing.push('ANTHROPIC_AGENT_ID')
  if (!ENVIRONMENT_ID) missing.push('ANTHROPIC_ENVIRONMENT_ID')
  if (missing.length) {
    return new Response(`Server is missing env var(s): ${missing.join(', ')}`, {
      status: 500,
    })
  }

  let client
  try {
    client = getClient()
  } catch (err) {
    return new Response(
      `Failed to initialize agent: ${err.message || err}`,
      { status: 500 },
    )
  }

  const userMessage = `Here is a new public media station intake:

- Station Name: ${stationName}
- Station Location: ${stationLocation}
- Station Website: ${stationWebsite}

Please follow your instructions to produce the funding outlook brief.`

  // ----- Create the session -----
  let session
  try {
    const sessionParams = {
      agent: AGENT_ID,
      environment_id: ENVIRONMENT_ID,
    }
    if (VAULT_IDS.length) sessionParams.vault_ids = VAULT_IDS
    session = await client.beta.sessions.create(sessionParams)
  } catch (err) {
    return new Response(
      `Failed to create session: ${err.message || err}`,
      { status: 500 },
    )
  }

  // ----- Open the event stream BEFORE sending the user message -----
  // (Stream-first ensures we don't miss any events the agent emits.)
  let initialUpstream
  try {
    initialUpstream = await client.beta.sessions.events.stream(session.id)
  } catch (err) {
    return new Response(
      `Failed to open session event stream: ${err.message || err}`,
      { status: 500 },
    )
  }

  // ----- Send the kickoff user.message event -----
  try {
    await client.beta.sessions.events.send(session.id, {
      events: [
        {
          type: 'user.message',
          content: [{ type: 'text', text: userMessage }],
        },
      ],
    })
  } catch (err) {
    try {
      initialUpstream.controller?.abort?.()
    } catch {
      /* ignore */
    }
    return new Response(
      `Failed to send user message: ${err.message || err}`,
      { status: 500 },
    )
  }

  const encoder = new TextEncoder()
  const seenEventIds = new Set()
  const deadline = Date.now() + RECONNECT_BUDGET_MS

  const body = new ReadableStream({
    async start(controller) {
      let lastSendAt = Date.now()
      const writeJson = (obj) => {
        try {
          controller.enqueue(encoder.encode(JSON.stringify(obj) + '\n'))
          lastSendAt = Date.now()
        } catch {
          /* controller closed */
        }
      }

      // Heartbeat to keep the downstream connection alive while we're
      // waiting on the next upstream event (e.g. during a long MCP tool
      // call or before the next model_request_start).
      const heartbeat = setInterval(() => {
        if (Date.now() - lastSendAt >= HEARTBEAT_MS) {
          writeJson({ type: 'heartbeat' })
        }
      }, HEARTBEAT_MS / 2)

      const handle = (event) => {
        switch (event.type) {
          case 'agent.message': {
            if (Array.isArray(event.content)) {
              for (const block of event.content) {
                if (block?.type === 'text' && block.text) {
                  writeJson({ type: 'text', text: block.text + '\n\n' })
                }
              }
            }
            break
          }

          case 'agent.thinking': {
            writeJson({ type: 'status', kind: 'thinking' })
            break
          }

          case 'agent.tool_use':
          case 'agent.mcp_tool_use': {
            writeJson({
              type: 'status',
              kind: 'tool_use',
              name: event.name || 'tool',
              label: describeToolUse(event),
            })
            break
          }

          case 'agent.tool_result':
          case 'agent.mcp_tool_result': {
            if (event.is_error) {
              const msg =
                (Array.isArray(event.content) &&
                  event.content[0]?.text) ||
                'tool error'
              writeJson({
                type: 'status',
                kind: 'tool_result',
                ok: false,
                message: String(msg).slice(0, 300),
              })
            } else {
              writeJson({ type: 'status', kind: 'tool_result', ok: true })
            }
            break
          }

          case 'session.error': {
            const msg = event.error?.message || 'unknown session error'
            writeJson({
              type: 'status',
              kind: 'session_error',
              message: msg,
            })
            break
          }
        }
      }

      let done = false
      let currentStream = initialUpstream
      let iteration = 0

      try {
        while (!done) {
          iteration++

          // On reconnect (every iteration after the first), reopen the
          // live stream and backfill anything we missed during the gap.
          if (iteration > 1) {
            if (Date.now() >= deadline) {
              writeJson({
                type: 'error',
                message: `Stream reconnect budget (${Math.round(
                  RECONNECT_BUDGET_MS / 60_000,
                )} min) exhausted. The session may still be running — try again or check the Console.`,
              })
              break
            }

            await new Promise((r) => setTimeout(r, RECONNECT_BACKOFF_MS))

            try {
              currentStream = await client.beta.sessions.events.stream(
                session.id,
              )
            } catch (err) {
              writeJson({
                type: 'error',
                message: `Failed to reopen event stream: ${err?.message || err}`,
              })
              break
            }

            // Backfill: pull everything the session has emitted so far
            // and dedupe by event.id. Auto-paginated by the SDK.
            try {
              for await (const event of client.beta.sessions.events.list(
                session.id,
                { order: 'asc' },
              )) {
                if (event.id && !seenEventIds.has(event.id)) {
                  seenEventIds.add(event.id)
                  handle(event)
                }
                // Terminal checks must run even for already-seen events,
                // or a terminal event that came in via the backfill gets
                // skipped and the loop never exits.
                if (isTerminal(event)) {
                  done = true
                  break
                }
              }
            } catch (err) {
              writeJson({
                type: 'status',
                kind: 'session_error',
                message: `Backfill failed: ${err?.message || err}`,
              })
              // Don't break — fall through and try the live stream.
            }

            if (done) break
          }

          // Tail the currently-open live stream until it ends, errors,
          // or hits a terminal session event.
          try {
            for await (const event of currentStream) {
              if (event.id && !seenEventIds.has(event.id)) {
                seenEventIds.add(event.id)
                handle(event)
              }
              if (isTerminal(event)) {
                done = true
                break
              }
            }
          } catch {
            // Treat as transient. If it's actually persistent, the
            // reopen + backfill on the next iteration will surface it
            // (or burn down the reconnect budget cleanly).
          }
        }

        if (done) {
          writeJson({ type: 'done' })
        }
      } catch (err) {
        writeJson({
          type: 'error',
          message: err?.message || String(err),
        })
      } finally {
        clearInterval(heartbeat)
        try {
          currentStream?.controller?.abort?.()
        } catch {
          /* ignore */
        }
        try {
          controller.close()
        } catch {
          /* ignore */
        }
      }
    },
    cancel() {
      // Browser disconnected. Best-effort: abort whichever upstream is
      // currently open. (`currentStream` lives inside `start()` so we
      // only have the initial one in scope here — that's enough to make
      // sure we don't keep a socket open after the very first turn.)
      try {
        initialUpstream.controller?.abort?.()
      } catch {
        /* ignore */
      }
    },
  })

  return new Response(body, {
    status: 200,
    headers: {
      'Content-Type': 'application/x-ndjson; charset=utf-8',
      'Cache-Control': 'no-cache, no-transform',
      'X-Accel-Buffering': 'no',
    },
  })
}

export const config = {
  path: '/.netlify/functions/agent-proxy',
}